Improve IsMovableObject logic.

The new logic says the main space is non movable iff neither the
background collector or foreground collector are compacting.
Temporarily fixes performance regressions and stability issues
caused by buggy JNI code.

Also addressed a few review comments from previous CLs.

Change-Id: Id5f91765b5fbdae387b566c0bda3392557218ba1
2 files changed